Characterization of dual beam optical tweezers system using a novel detection approach

Characterization of optical tweezer system involves determining the stiffness coefficient of the trap and the range in which it behaves as a linear spring. This is often done by calculating the power spectrum of the fluctuations of the trapped bead observed on a photodetector. The detection process entails observing the deflection of either the same trapping beam or a different laser beam of very low power. We present a new approach where a laser beam from a source is split into s- and p- polarizations where s-polarization is used for trapping and p-polarization for detection. We also analyze and experimentally verify the effect of the detection beam on the trap dynamics. We further explain how to incorporate the effect of a strong detection beam, capable of trapping beads, in the photodetector calibration in a dual beam trap scenario. The novel detection method and the associated modelling sets the stage for the application of modern control approach.